Add pull request template

Signed-off-by: Michal Čihař <michal@cihar.com>

.github/PULL_REQUEST_TEMPLATE.md

+Before submitting pull request, please ensure that:
+
+- [ ] Every commit has message which describes it
+- [ ] Every commit is needed on it's own, if you have just minor fixes to previous commits, you can squash them
+- [ ] Any code changes come with testcase
+- [ ] Any new functionality is covered by user documentation